The present invention relates to the art of x-ray tube control. It finds particular application in conjuntion with light weight, portable x-ray systems and will be described with particular reference thereto. However, it is to be appreciated that the present invention may also find application in other x-ray systems and other control applications, particularly those in which large amounts of electrical power are controlled with precision.
Most x-ray systems are designed for a fixed installation. Because the characteristics of electrical power available to the unit are known, the unit is constructed with appropriate components. Some systems are designed to accomodate either of two line voltages, such as either 220 or 440 volts. The multiple line voltage systems include an appropriate step-up or step-down transformer with multiple taps to convert either line voltage level to a preselected internal operating voltage. However, transformers, particularly transformers which handle the large amounts of power required by an x-ray system are heavy. The weight is particularly disadvantageous in a portable system.
Adapting an x-ray system to operate on single phase versus three phase current or vice versa is more difficult. Commonly, it is necessary to replace the whole power module. For a portable system, carrying multiple power modules again adds weight and requires additional space. Further, replacement of the modules with each move requires additional man-power and time to set up the system.
Most commonly, x-ray systems employ silicon controlled rectifiers to switch power to the x-ray tube at a relatively low frequency. One drawback of SCR switching systems is that they require bulky commutation circuitry to turn the devices off once energized. Moreover, radical load variations can cause miscommutation. Varying loads can affect the circuit characteristics of SCR switched systems reducing the dynamic output voltage range. Because gate turn-off thyristors require large gate currents to turn off, complex gate drive circuitry is required.
Some x-ray generators have been provided which have transistor switching. Often, the switching frequency of the transistors is varied to vary the output voltage by using the resonant characteristics of the load. However, generators that change pulse repetition rate tend to exhibit significant ripple amplitude variations in the output x-ray beam.
In accordance with the present invention, a new and improved high voltage control circuit is provided which overcomes the above referenced problems and others.